The quasi-synchronous detection typically used as a detection method in landline mobile communications requires that a transmission carrier frequency matches a reference carrier frequency for quasi-synchronous detection of a receiver. However, when oscillators in a transmitter and a receiver are not sufficiently high in frequency stability and accuracy, this results in a difference in frequency between both sides. This is called a “frequency offset.” Since the frequency offset, if any, causes the phase of a detected signal to rotate, the signal cannot be correctly demodulated. To prevent this incorrect demodulation, AFC (Automatic Frequency Control) is typically used. The AFC estimates a frequency offset in carrier frequency between the transmission side and reception side to control the frequencies of oscillators. The AFC also corrects demodulated signals for the rotated phase caused by the frequency offset.
Conventionally, a frequency offset estimator for use in the AFC relies on the differential detection as can be seen in FIG. 2 in Laid-open Japanese Patent Application No. 8-213933, “Characteristics of ½ Symbol Differential Detection AFC Having wide Frequency Pull-in Range,” Technical Report, RCS96-25, 1st-6th Paragraphs, June 1996, “Frequency Offset Estimating Method in Fading Transmission Path with Large Time Dispersion,” Technical Report, RCS98-81, 13th-18th paragraphs, September 1998, and the like. However, the differential detection type frequency offset estimator has a drawback that the accuracy of estimation is significantly degraded when the carrier to noise power ratio (CNR) is low.
For example, a CDMA (Code Division Multiple Access) communication system using the quasi-synchronous detection effectively utilizes a path diversity effect resulting from rake combination. In addition, required SNR (Signal to Noise Power Ratio) may occasionally be on the order of 0 dB at a bit error rate (BER) of 0.1%, resulting from the effects of error correcting codes, transmission power control and the like. It is therefore necessary to provide an expedient which is capable of estimating a frequency offset even at low CNR.
A conventional frequency offset estimator will be described with reference to FIG. 1. The differential detection frequency type offset estimator shown in FIG. 1 estimates a frequency offset from a difference in phase between symbols. First, complex multiplier 3 calculates a product of orthogonally detected complex demodulated symbol sequence 1 and a complex conjugate of a known symbol sequence 2 corresponding thereto. The product is fed to differential detector 15 as complex symbol sequence 13. Differential detector 15 delays complex symbol sequence 13 by several symbols using delay circuit 16, and complex multiplier 3 calculates a product of a complex conjugate of the delayed symbol sequence, which have passed through delay circuit 16, and original complex symbol sequence 13. This product is provided to averaging circuit 17 for averaging, and then delivered as frequency offset estimate 18. In this event, as larger noise is added to orthogonally detected complex demodulated symbol sequence 1, larger variations occur in the phase difference between symbols, resulting in a degraded accuracy of estimation for frequency offset. It is known that the accuracy of estimation is improved to some extent even using the differential detection, if the number of delayed symbols is increased. This is because the phase difference between symbols becomes larger relative to variations in phase due to noise. Disadvantageously, however, an increased number of delayed symbols results in a narrower range in which a frequency offset can be estimated. Therefore, when the differential detection type frequency offset estimator is used, a tradeoff is inevitably made between the accuracy of estimation and an estimatable range in regard to the number of delayed symbols.
On the other hand, there has been proposed an FFT (Fast Fourier Transform) based method as another frequency offset estimating method. This type of estimating method converts received symbols into a frequency domain by FFT, and determines a frequency indicative of a peak of a spectrum envelope as a frequency offset. This estimating method provides a higher accuracy of estimation at low CNR than the differential detection since the peak can be relatively easily found even if a received signal presents low CNR. However, the accuracy of estimation depends on the order of FFT. An article “FFT-Based Highly Accurate Frequency Determining Method,” Transactions-A of the Institute of Electronics, Information and Communication Engineers, Vol. J 70-A, No. 5, pp.798–805 describes that FFT should be used at 32 points or more for estimating a frequency using FFT. However, the FFT cannot be used at 32 points or more in some occasions.
Conventionally, known symbols transmitted in a predetermined order have been used for estimating a frequency offset. In mobile communications, a section comprised of several symbols is called a “slot” which contains pilot symbols, data symbols, control symbols and the like. The pilot symbols refer to known symbols which are transmitted in a predetermined order. While the total number of symbols in a slot ranges from about fifteen to several hundreds, the number of pilot symbols is generally smaller than the number of data symbols.
Taking as an example, the international standard IMT-2000 for the next generation mobile communications, as described in an article “3G TS 25.211 version 3.2.0, 3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD),” one slot includes only 16 pilot symbols at most even at a high bit rate. In other words, when a frequency offset is estimated using sequential pilot symbols in one slot interval, a sufficient number of pilot symbols is not provided for utilizing the FFT. When pilot symbols are used over a plurality of slot intervals, a frequency offset can be estimated in a narrower range.
On the other hand, a peak detection type frequency offset estimating method has also been proposed, as can be seen in FIG. 1 in Laid-open Japanese Patent Application No. 9-200081. The proposed frequency offset estimating method, which is for use in a direct code spread communication system, involves despreading baseband complex signals orthogonally detected using complex spread codes previously applied with frequency offsets which have an equal absolute value and different signs, averaging several symbols acquired at timings at which a maximal peak is detected, and converting the power value of the average to a frequency offset using a previously measured conversion table. This estimating method is considered to provide a better accuracy of estimation at low CNR than the differential detection type since it uses an average value of symbols which are despread by spreading codes applied with frequency offsets. However, a conversion table must be previously created for calculating a frequency offset. When the conversion table is used, it can be thought that if the characteristics of devices vary from one apparatus to another, a resulting frequency offset may be different depending on a particular apparatus, so that the creation of a conversion table, in general, is not an easy task. In addition, a memory is required for storing the conversion table. Moreover, a correction may be required for suppressing variations in the characteristics of devices between apparatuses.